This invention relates generally to cable clamps for clamping electrical cables in overhead transmission lines and particularly to a low cost side opening cable clamp that is simple to manufacture and easy to use.
Power line installation and maintenance crews prefer cable clamps that can be installed without removing the clamping hardware or cutting the associated cable. Side opening cable clamps are available in two different types, commonly referred to as a straight line type and a quadrant type. The straight line type is preferred, especially when the clamp is being installed on an energized high voltage overhead line, since it is much simpler to manipulate and doesn't require disassembly of the clamp or cutting of the cable. Preferably, a cable clamp should be relatively simple to install and have captivated parts which can not be disengaged from the clamp during installation.
In a conventional straight line, side opening cable clamp, a body is provided with a cable receiving means or groove, a keeper to enable the cable to be physically clamped in the cable receiving groove, one or more U bolts to secure the keeper to the body, a clevis for attaching the cable clamp to a supporting structure, such as a power insulator string, and a pulling eye to tension the cable in position in the cable receiving groove prior to clamping the cable with the keeper.
U.S Pat. No. 4,383,668 discloses a side opening cable clamp having a pair of captivated U bolts for securing a keeper that is normally urged to a position away from a cable supporting groove by means of a pair of guide rails and a compression spring. The guide rails extend upwardly from the edges of the cable supporting groove in the body and essentially balance the keeper over the cable during installation. The keeper movement is restrained to a straight line path by the guide rails. To avoid friction problems, which can result in variable clamping forces on the cable, the U bolts must be alternately tightened. Thus the lineman needs to shift from one U bolt to the other a number of times during the installation. This becomes burdensome when using long handled hot line tools to install a clamp on an energized high voltage power line. Of even more importance is the fact that the spring encircles an upstanding post in the body and nests in a deep recess in the keeper. The post, the recess and the guide rails are difficult to cast in a mold and render the clamp design considerably less attractive from an economic standpoint.
U.S. Pat. No. 3,470,528 discloses a side opening dead end clamp in which the keeper may be swung out, pivoting on one U bolt leg, to expose the cable supporting groove of the cable clamp. This clamp does not include means for automatically opening the clamp, which is a very desirable feature, especially when the cable clamp is being installed on an energized transmission line. The pattern of movement of the keeper is somewhat complicated and the clamp is consequently comparatively difficult to install.
U.S. Pat. No. 2,997,764 discloses a quadrant type cable clamp in which the keeper is pivotally mounted to the body by means of a wire loop spring that serves to automatically open the clamp as the U bolt is released. Quadrant type clamps are much more difficult to install because the cable must be manipulated around the clamp. This means that the cable must either be cut and fed through the clevis or the clevis pin removed from the clamp.
Cable clamps as described above are conventionally used to clamp ACSR type cable that is composed of multiple strands of aluminum wire wound over a center core of one or more galvanized steel strands. A typical failure mode of such cable clamps is that the interior steel core of the ACSR cable will slip in time, which causes breaking of the outer aluminum strands. In U.S. Pat. No. 3,602,956, a suspension type cable clamp mechanism is disclosed in which the forces exerted by the keeper on the cable are examined. The thrust of the patent is that the compressive force on the cable should be moderated in accordance with the tension force in the cable to preclude the development of excessive shear stress in portions of the cable.
A straight line, side opening, single U bolt cable clamp, that is generally constructed like the two U bolt version discussed in the '688 patent, is commercially available. In it, the keeper movement is confined to a straight line by the guide rails opposite the cable supporting groove. While it is self-opening, the shortcomings described above with respect to the 688 patent clamp parting line still exist. Further as the single U bolt is tightened to secure the cable, the keeper is caused to bear heavily against the rails. This results in a frictional force on the keeper that can greatly reduce the actual clamping force that is applied to the cable. A wax lubricant is placed on the rails by the manufacturer to limit this friction. The wax lubricant is however subject to removal before the clamp is installed and consequently there is a significant variability in the holding power of the clamp. Further, the lubricant is eventually lost due to exposure to the elements and limits the reusability of the clamp.
The present invention cable clamp is a novel straight line, side opening device that incorporates features of the above prior art cable clamps in a unique way to produce a clamp that; is readily manufacturable at low cost, easy to install under energized line conditions, avoids overstressing the cable, and provides better reliability.